Japanese Examined Patent Publication (kokoku) No. 155215/1987 reports that a kind of pyridine compound can be used as a potentiator of antitumor effect of 5-fluorouracil (hereinafter referred to as "5-FU") and derivatives of 5-FU. The pyridine compound has the feature of sustaining the concentration of 5-FU in the living body. On the other hand, 5-FU is known to have the serious problem that the prolonged presence of 5-FU in the living body is likely to cause a disorder (inflammation) in the oral cavity, gastrointestinal tissue and the like as often experienced in continuous intravenous infusion of 5-FU alone.
International Publication WO 90/07334 reports that oxonic acid used in combination with 5-FU or 5-FU derivative can inhibit the occurrence of inflammation caused by 5-FU or 5-FU derivative. Yet oxonic acid in this case reduces the antitumor effect of 5-FU or 5-FU derivative. Thus oxonic acid is not satisfactory in terms of potentiation of antitumor effect and alleviation of adverse effects.